Ion source for time-of-flight mass spectrometer

ABSTRACT

A method and apparatus for eliminating unwanted signals (noise) at a mass spectrometer output by improving the formation of ions in a mass spectrometer with a pulse forming network which applies voltage pulses to the control electrode and the backing plate and accelerating electrode of the ionization region. The pulses are oriented in time so that voltages are approximately zero at the backing plate and accelerating electrode during the presence of voltage pulses at the control electrode which accelerate electrons into the ionization region. The magnitudes of the pulses applied to the backing plate and accelerating electrode are such that an electric field is periodically established therebetween to accelerate ions from the ionization region with the durations of the pulses oriented in time so that the electric field established by the pulses has a duration less than the duration of the negative voltage pulse applied to the backing plate.

United States Patent [72] Inventor William H. Shriner Blanchester, Ohio [21] Appl. No. 863,880

[22] Filed Oct. 6, 1969 [45 Patented Nov. 9, 1971 [73] Assignee The Bendix Corporation [54] ION SOURCE FOR TIME-OF-FLIGHT MASS OTHER REFERENCES Primary ExaminerWilliam F. Lindquist Attorneys-Raymond J. Eifler and Flame, Arens, l-lartz, Hix

and Smith ABSTRACT: A method and apparatus for eliminating unwanted signals (noise) at a mass spectrometer output by improving the formation of ions in a mass spectrometer with a pulse forming network which applies voltage pulses to the control electrode and the backing plate andaccelerating electrode of the ionization region. The pulses are oriented in time so that voltages are approximately zero at the backing plate and accelerating electrode during the presence of voltage pulses at the control electrode which accelerate electrons into the ionization region. The magnitudes of the pulses applied to the backing plate and accelerating electrode are such that an elec tric field is periodically established therebetween to accelerate ions from the ionization region with the durations of the pulses oriented in time so that the electric field established by the pulses has a duration less than the duration of the negative voltage pulse applied to the backing plate.

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SHEET 1 OF 2 NETWORK 68 OSCI LLOSCOPE FIGURE WILLIAM H. SHRI NER INVENTOR.

BY Wag-51% PATENTEUIIIII 9 WI SHEET 2 OF 2 POWER suppmsz NEGATIVE VOLTAGE INPUT (5 OUTPUT B TO BACKING PLA 9.- 5 Il II My m/ m 4 R 5%. wow PTC N IO A M R O Tm .IT S wm WAE FIGURE 3 l TIME F-TIME TIME WILLIAM H. SHRINER INVENTOR.

FIGURE 2 TTORNEY CONTROL ELECTRODE G E MD 0 R LT EC CF- CL AE ION souncs non rmn-or-strcn'r MASS SPECTROMETER BACKGROUND OF THE INVENTION The invention relates to mass spectrometers and more specifically to ion sources.

The mass spectrometer is an instrument that permits rapid analysis of molecular species by measurement of the masses of the different ions after ionization of the molecules. In operation, a small amount of gas to be analyzed is admitted through a sample inlet into an ionization chamber or region where the gas is ionized by electrons emitted from a filament. The ions are then directed or accelerated by an electric field from the ionization chamber and into a region where the ions are separated according to their mass to charge ratio (m/e). The ions then impinge upon the cathode of an electron multiplier to achieve a gain of or greater. The resulting output signal is then coupled to a device which indicates the mass spectrum of the particular gas. The spectrum indicates the elements and/or molecules which make up the gas. For accurate analysis i.e. quantitative and qualitative, it is essential that unwanted signals (noise) are eliminated. In a time of flight mass spectrometer and especially those of the type utilizing time-lag energy focusing (See Time of Flight Mass Spectrometer with Improved Resolution Volume 26 Number 12 of the Review of Scientific Instruments, Dec. 1955) the elimination or reduction of noise is a necessity if accurate analysis is to be obtained. A major source of noise in the aforementioned type of mass spectrometer is the ion source which contains ions which have initial space and initial kinetic energy distributions inconsistent with such distributions on the majority of ions.

SUMMARY OF THE INVENTION To eliminate noise in a time of flight mass spectrometer, especially when utilizing time-lag energy focusing, a negative bias is applied to the backing plate of the ionization region during the interval between voltage pulses applied to the control electrode, which accelerates electrons into the ionization region and before and during the pulses applied to the accelerating electrode which establishes an electric field that accelerates ions from the ionization region.

The invention is characterized by the fact that before each group of ions is accelerated from the ionization region by a negative potential applied to the accelerating electrode, a negative potential is applied to the backing plate and that during the acceleration of electrons into the ionization region by the control electrode no negative potential is present at the backing plate. In one embodiment of the invention this is accomplished by a synchronized multiple pulse forming network that applies different pulse trains having the same period (T) to the control electrode, which accelerates electrons into the ionization region, to the backing plate, and to the accelerating electrode, which accelerates the ionized molecules from the ionization region, in a manner that provides the absence of a negative voltage at the backing plate and accelerating electrode during the presence of a positive voltage at the control electrode, and the presence of a negative voltage at the backing plate after the termination of the negative voltage applied to the accelerating electrode. The resultant pulse trains are further characterized by the fact that the pulses applied to the accelerating electrode are pulses of greater magnitude but shorter duration than the pulses applied to the backing plate.

Accordingly, it is an object of this invention to improve the accuracy of a time of flight mass spectrometer which utilizes timelag ion focusing.

It is another object of this invention to remove ions remaining in an ionization region after the acceleration of ions from the ionization region and before the formation of new ions in that region by a pulse ofelectrons accelerated therein.

It is still another object of this invention to provide a method for forming and accelerating ions for analysis in a muss spectrometer.

It is u further object of this invention to provide an improved ion source.

It is still a further object of this invention to provide a multiple pulse forming network for accelerating electrons into and ions out of an ionization region with a minimum amount of noise and minimum loss of ions.

The above and other objects and features of the invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings and claims which form a part of this specification.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a diagrammatic view of a mass spectrometer show- DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawings, FIG. 1 illustrates a mass spectrometer of the type described in U.S. Pat. No. 2,797,330 which utilizes the principles of the invention. A filament I0, made of a material which emits electrons when heated, is located adjacent a control electrode 12. The control electrode 12 has a slot M to permit the passage of electrons emitted from the filament into an ionization region 23 between a backing plate 22 and an accelerating electrode 24. A collector 20 located on the other side of the ionization region 23 collects electrons emerging from the ionization region 23. A sample to be analyzed is located in receptacle 34 from which it is transferred in the gaseous state through conduit 32 into the ionization region. Accelerating electrode 24 located parallel to the backing plate 22 has a slot 26 to permit the passage of ions from the ionization region 23 into an analyzing region 27 where the ions are separated according to their mass to charge ratio (elm). A detector or ion collector 40, connected to an oscilloscope 42 collects the ions and gives an indication of the relative masses of the ions collected. A pulse network 68 is connected to the backing plate 22, the accelerating electrode 24, and the control electrode 12 to supply the necessary voltages to accelerate the electrons into the ionization region 23, to accelerate ions out of the ionization region 23, and to clear the region of stray ions after those functions occur. A voltage pulse is also applied to the oscilloscope 42 to synchronize the oscilloscope with the pulses applied to the backing plate 22 and accelerating electrode 24. The characteristics of, and a representation portion of the pulse forming network 68 is shown in FIGS. 2 and 3. Examples of other circuits which may be adapted for use are the Double Pulse Generator model 903 manufactured by the Berkeley Scientific Instrument Company of Richmond California, (produces a plurality of pulses separated from each other by variable time periods) and the pulse forming circuit as disclosed in abandoned application 288,104, filed May 16, 1952, by Macon H. Miller and William C. Wiley. A power supply 52 supplies the necessary potentials to the pulse network 68, the cathode l0, electron collector 20, and ion detector 40. Where desirable a capacitor and/or resistor may be used to couple the different components together or to ground.

FIG. 2 is a graphical representation on the same time scale of voltages which appear at the backing plate, accelerating electrode and control electrode to achieve the objects of the invention. Curve A is a positive voltage pulse train having a period T with positive pulse durations of t occurring after time intervals of t, and is applied to the control electrode. During time t, the control electrode voltage is preferably negative to prevent the passage of electrons into the ionization region. Curve B is a negative voltage pulse train having a period T with pulse durations t occurring after time intervals of t and is applied to the backing plate. Curve C is a negative voltage pulse train having a period T with pulse durations t occurring after time intervals of t and is applied to the accelerating electrode. The magnitude of the pulses shown in Curve C are sufiiciently greater (preferably 10 times greater) than the pulses shown in Curve B so that an electric field is established during time intervals t between the backing plate and accelerating electrode to accelerate ions from the ionization region. When time lag ion focusing is employed there is a time delay 7 of approximately 2.5 microseconds before the application of a pulse to the accelerating electrode (Curve C) after the application of a pulse to the backing plate (Curve B). It should be noted at this point that pulse trains may be obtained in various ways such as periodically combining a voltage of one polarity with a voltage of opposite polarity to achieve a pulse train having pulses of the desired magnitude, duration, and polarity. For example the negative voltage pulse train shown in Curve B can be obtained by combining a continuous negative bias with positive pulses of the same magnitude as the negative bias.

FIG. 3 is a preferred embodiment of a circuit for applying voltages to the backing plate (Curve B FIG. 2) in response to voltage pulses applied to the control electrode (Curve A). The circuit shown applies a negative bias to the backing plate and removes the bias during the time interval that a positive voltage pulse is present at the control electrode. In the circuit shown input E receives pulses as shown in Curve A, FIG. 2 from a pulse generator (not shown). Circuit inputs F and G are connected to the positive and negative terminals of a power supply respectively. Circuit outputs A and B are connected to the control electrode and backing plate respectively and generate the wave forms shown in Curves A and B of FIG. 2 respectively. Input E is connected to a capacitor 2, which is connected to output A, and a resistor 3 which is connected in series to a variable resistor 4. The other end of the variable resistor 4 is connected to common through a capacitor 1 while the wiper arm of the resistor 4, which picks off the voltage applied to the backing plate, is connected to output B through a capacitor 5. The position of the wiper arm on resistor 4 can be varied to adjust the bias level applied to the backing plate. Input G is connected to output A and capacitor 2, through an electrically parallel resistor 8 and diode 9. Input F is connected through a resistor 6 to output B, a capacitor 5, and a diode 7 which is connected to common. When the circuit is energized and in the absence of a positive voltage pulse applied to input E, a negative bias is maintained at the backing plate through output B. This is accomplished by locating the wiper arm on resistor 4 so that the wiper arm picks up a positive voltage greater than +0.5 volts. When a positive voltage pulse is applied at input E diode 7 conducts for the duration of the pulse causing the backing plate voltage level at output B to drop to approximately zero (+0.5 volts which is the voltage across diode 7). In one preferred embodiment the following circuit elements and values were used to obtain the desired pulse outputs: Capacitor 1, l microfarads; capacitor 2, 0.02 microfarads; resistor 3, 22 ohms; variable resistor 4, 50 ohms; capacitor 5, 0.02 microfarads; resistor 6, 51K ohms; diode 7, 1N9l4; resistor 8, 10K; diode 9, lN9l4 and +30 volts applied to input F.

OPERATION When utilized in a mass spectrometer the invention operates in the following manner. A small amount of gas to be analyzed introduced into the ionization region 23 where the gas molecules are ionized by electrons. The electrons emitted by filament 10, are periodically (e.g. every to 50 microseconds) accelerated into the ionization region by the application of a positive pulse train to the control electrode. FIG. 2, Curve A, is representative of the pulse train applied to the control electrode 12 which preferably has a repetition rate of 20,000 to 50,000 pulses per second depending upon the type of spectrometer. Simultaneously, but out of phase with the pulses applied to the control electrode, pulses of the same repetition rate are applied to the accelerating electrode 24 to accelerate each group of molecules ionized by the electrons out of the ionization region 23 and into the analyzing region 27. FIG. 2, Curve C is representative of the pulse train applied to the accelerating electrode 24 and shown the relationship in time of the pulses applied to the control electrode 12 and the accelerating electrode 24. To clear ions from the ionization region 23 between each pulse of electrons into the region a negative voltage is applied to the backing plate 22. However, it is important that the negative voltage applied to the backing plate 22 be removed during the application of a positive voltage to the control electrode 12 otherwise a severe loss of ions in the lower mass range is experienced. FIG. 2, Curve B, is representative of the pulse train applied to the backing plate and shows the interrelationship in time of the control electrode l2, backing plate 22 and accelerating electrode 24.

The ions accelerated by a pulse applied to the accelerating electrode 24 travel through slot 26 and into the analyzing region 27 to ion collector 40. During their travel through the analyzing region 27, the ions become materially separated according to the mass to charge ratio. By measuring the relative times at which different groups of ions reach the collector 40, the masses of ions in each group can be determined. The oscilloscope 42 is one device which can be adapted to visually indicate the mass spectrum of the particular gas. Because of the clearing of ions from the ionization region 27 between the formation and acceleration of each group of ions resolution of the mass spectrum at the oscilloscope 42 is improved. In the absence of a negative potential applied to the backing plate in the manner previously described ions not accelerated from the ionization region when a pulse is applied to the accelerating electrode 24 are accelerated from the ionization region 23 by a subsequent pulse. However, since these ions were previously subjected to an electric field their initial space and initial kinetic energies are different from those ions which have just been formed. Since the spectrometer cannot discriminate against these ions which are in a different reference frame, they strike the ion detector 40 and appear as lines on the mass spectrum. These lines are false indications of the presence of a mass and are indistinguishable from those mass lines which are properly present.

While a preferred embodiment of the invention has been disclosed, it will be apparent to those skilledin the art that changes may be made to the invention as set forth in the appended claims, and, in some cases, certain features of the invention may be used to advantage without corresponding use of other features. For example, the magnitudes, durations and polarities of the pulses applied to the control electrode, backing plate, and accelerating electrode may be combined in different ways to achieve the objects of the invention. Further, ions remaining in the ionization region, after a pulse has been applied to the accelerating electrode, may be removed by an electric field which is established by applying a potential to a component or plate other than the backing plate of the ionization region. Accordingly, it is intended that the illustrative and descriptive materials herein be used to illustrate the principles of the invention and not to limit the scope thereof.

Having thus described the invention, what is claimed is:

1. In combination with a time-of-flight mass spectrometer of the type having, a drifl tube, an ion collector, and an ion source of the type wherein electrons emitted from a filament are accelerated by a control electrode into an ionization region between a backing plate and an accelerating electrode and wherein a gas is introduced into the ionization region and ionized by the electrons therein and wherein ions are accelerated from the ionization region by an electric field between the backing plate and accelerating electrode, the improvement which comprises:

means for applying a positive voltage pulse train to the control electrode to accelerate electrons into the ionization region, said pulses having a period (T) and discrete time durations (u);

means for applying a negative voltage pulse train to the accelerating electrode to accelerate ions from the ionization region, each of said accelerating electrode pulses having a discrete time duration (t between said control electrode pulses; and

means for applying a negative voltage pulse train to the backing plate to remove residual ions from said ionization region except during time period (t,) when ions are accelerated from said ionization region, each of said backing plate pulses having a discrete time duration (t,) between said control electrode pulses so that no negative voltage is present at the backing plate during the presence of a positive voltage pulse at the control electrode.

2. The combination as recited in claim 1 wherein the time durations (t;,) of said pulses applied to the acceleration electrode is less than the time durations (t,) of said pulses applied to the backing plate.

3. The combination as recited in claim 2 wherein the time durations (t;,) of said pulses applied to the accelerating electrode occurs simultaneously with said pulses applied to the backing plate.

4. The combination as recited in claim 3 wherein the time duration (t,,) of a pulse applied to the backing consists of the entire time interval between said pulses applied to the control electrode.

5. The combination as recited in claim 4 wherein said pulses applied to the accelerating electrode have a magnitude greater than the magnitude of said pulses applied to the backing plate.

6. The combination as recited in claim 5 wherein a pulse applied to the accelerating electrode occurs at least I microsecond after a pulse is applied to the backing plate.

7. The combination as recited in claim 4 wherein the magnitude of said pulses applied to the accelerating electrode is at least 10 times greater than the magnitude of said pulses applied to the backing plate. 

1. In combination with a time-of-flight mass spectrometer of the type having, a drift tube, an ion collector, and an ion source of the type wherein electrons emitted from a filament are accelerated by a control electrode into an ionization region between a backing plate and an accelerating electrode and wherein a gas is introduced into the ionization region and ionized by the electrons therein and wherein ions are accelerated from the ionization region by an electric field between the backing plate and accelerating electrode, the improvement which comprises: means for applying a positive voltage pulse train to the control electrode to accelerate electrons into the ionization region, said pulses having a period (T) and discrete time durations (t1); means for applying a negative voltage pulse train to the accelerating electrode to accelerate ions from the ionization region, each of said accelerating electrode pulses having a discrete time duration (t3) between said control electrode pulses; and means for applying a negative voltage pulse train to the backing plate to remove residual ions from said ionization region except during time period (t3) when ions are accelerated from said ionization region, each of said backing plate pulses having a discrete time duration (t2) between said control electrode pulses so that no negative voltage is present at the backing plate during the presence of a positive voltage pulse at the control electrode.
 2. The combination as recited in claim 1 wherein the time durations (t3) of said pulses applied to the acceleration electrode is less than the time durations (t2) of said pulses applied to the backing plate.
 3. The combination as recited in claim 2 wherein the time durations (t3) of said pulses applied to the accelerating electrode occurs simultaneously with said pulses applied to the backing plate.
 4. The combination as recited in claim 3 wherein the time duration (t2) of a pulse applied to the backing plate consists of the entire time interval between said pulses appliEd to the control electrode.
 5. The combination as recited in claim 4 wherein said pulses applied to the accelerating electrode have a magnitude greater than the magnitude of said pulses applied to the backing plate.
 6. The combination as recited in claim 5 wherein a pulse applied to the accelerating electrode occurs at least 1 microsecond after a pulse is applied to the backing plate.
 7. The combination as recited in claim 4 wherein the magnitude of said pulses applied to the accelerating electrode is at least 10 times greater than the magnitude of said pulses applied to the backing plate. 